1. Field of the Invention
The present invention relates to compositions for controlling the gelation rates of water soluble polymer solutions used as fracturing fluids in well fracturing operations and, more particularly but not by way of limitation, to suspensions of anhydrous boron compounds in a mixture of mineral spirits (commonly termed as naphtha) and a resin or a mixture of an oil and a resin. The suspension provides a stable, non-aqueous, pumpable, liquid source of borate ions suitable for use as a gelling agent for water soluble polymer solutions.
2. Description of the Related Art
To perform a hydraulic fracturing operation, a proppant-laden water soluble polymer hydraulic fracturing fluid such as guar, hydroxypropyl guar (HPG), carboxymethyl guar (CMG), or carboxymethyl hydroxypropyl guar (CMHPG) may be injected under high pressure into a formation through a well bore. Once the natural confining pressures of the formation rock are exceeded, the fracturing fluid initiates a fracture in the formation rock that generally continues to grow during pumping. Hydraulic fracturing of the formation typically requires the fracturing fluid to reach its maximum viscosity as it enters the fracture. Increased viscosity of the fracturing fluid, which improves its ability to fracture the formation rock, usually occurs through the gelling of the water soluble polymer solution utilized as the hydraulic fracturing fluid. Gelation of water soluble polymer solutions may be achieved by the addition of aluminum, boron, titanium, or zirconium ions, or mixtures thereof to the solution.
However, if the fracturing fluid gels within the well bore, it encounters a high shear due to the limited cross-sectional area within the well bore. High shear experienced in the well bore may cause extensive and irreparable degradation in the cross-linked fracturing fluid. Furthermore, high viscosities in the fracturing fluid produce excessive back or friction pressures within the well bore and formation, thereby limiting the pumping rate and possibly the success of the hydraulic fracturing operation. Various borate ion cross-linking systems have been developed which delay the gelation of the fracturing fluid during its pumping through the well bore.
One such borate ion cross-linking system is disclosed in U.S. Pat. No. 4,619,776 issued on Oct. 28, 1986, to Mondshine. Mondshine discloses a suspension of alkaline earth metal borates, alkali metal borates, or mixtures thereof. The boron minerals are typically suspended in diesel oil and are sparingly soluble in the water soluble polymer solutions. After the introduction of the Mondshine borate suspension into a water soluble polymer solution, the suspension slowly thins and dissipates to release borate ions to gradually cross-link the polymer solution. That is, the slow solubility of the suspended, sparingly soluble borate minerals creates a cross-linking system that delays the transformation of polymer solutions into gelled, highly viscous fracturing fluids.
Although the borate ion cross-linking system disclosed by Mondshine will delay the gelation of water soluble polymer solutions, it suffers from numerous disadvantages. First, the suspension of the borate minerals in diesel oil creates a potential environmental hazard. Toxic aromatics and benzene derivatives contained in the diesel oil might contaminate marine environments or ground water sources that may be proximate to the diesel borate suspensions at well sites. Second, the utilization of borate minerals decreases the effectiveness of the suspension. Impurities in the boron minerals reduce the quantity of boron in the suspension which produces corresponding lower levels of borate ions upon the dispersion of the suspension in a water soluble polymer solution. Third, the impurities in the borate minerals introduce variability in the delay time. The exact solubility of the borate minerals are not determinable with a high degree of accuracy and vary widely depending upon the conditions of use. Fourth, the borate mineral suspensions are usable only in cases where the bottom hole temperature of the formation is higher than about 150.degree. F. That limitation occurs due to the very low solubility of the borate minerals and the excessive thermal delay in their cross-linking action. Finally, and perhaps most importantly, the borate mineral suspensions disclosed in Mondshine are highly unstable in suspension. That is, the suspension of the borate minerals in diesel oil may not be stored for any length of time and is highly susceptible to settling. It has been observed that the borate minerals, even after even a short length of time in storage settle to the bottom of the container in which they are stored and agglomerate into a solid. When the borate minerals cake to form a solid, the suspension becomes unusable. Even considerable shaking will not cause the borate minerals to become re-suspended. This tendency to settle may result in considerable loss of usable material.
An alternative borate ion cross-linking system is disclosed in U.S. Pat. Nos. 5,082,579; 5,145,590; and 5,160,643 issued on Jan. 21, 1992; Sep. 8, 1992; and Nov. 3, 1992, respectively, to Dawson. These patents disclose a borate ion based aqueous complexor solution that delays the gelation of water soluble polymer solutions. The aqueous complexor solution consists of a cross-linking additive that provides borate ions and a delay additive in solution that serves to chemically bond with the borate ions to reduce the availability of boron to the hydrated polymer solution.
Although the Dawson borate ion cross-linking system provides improvements to some of the deficiencies characteristic of Mondshine's teaching, such as lack of control over the gelation rate, Dawson's system still suffers from a number of disadvantages. The amount of available borate ions in solution may be insufficient to cross-link all of the polymer in solution due to the presence of the delay additive. The dilution of borate ions caused by the presence of the delay additive creates a demand for considerably higher quantities of borate ions than required in a stoichiometrically balanced system. The requirement for additional boron in excess of that necessary for a stoichiometrically balanced system increases the cost of the Dawson borate ion cross-linking system. A further economic disadvantage of the Dawson system is caused by the undesirable necessity of using large quantities of carbonate buffers, such as potassium carbonate, to obtain effective delay and temperature stability of the fracturing fluid. Furthermore, due to the presence of the delay additive in solution with the borate ions, the cross-linking action of the complexor solution is thermally delayed. That thermal delay restricts use of the Dawson system to formations having bottom hole temperatures higher than 125.degree. F. Finally, and perhaps most importantly, during storage, the borate ions in Dawson's complexor solution precipitate rendering the complexor solution deficient in boron.
Accordingly, any borate ion cross-linking system that is stable during storage, especially under severe weather conditions, yields 100% of its available boron to soluble borate ions, provides a large operational temperature range, and a wide range of cross-link delay times while retaining precise control of the specific delay is highly desirable.